Radio Frequency (RF) amplifiers are used in a variety of devices, including mobile communications devices such as mobile telephones. In particular, an RF transmit amplifier, also sometimes referred to as a power amplifier (PA) or RF PA, is employed to amplify and transmit an RF signal from a mobile communication device. There are a number of different performance criteria which apply to RF transmit amplifiers in mobile communication devices, including efficiency, power output level, gain, linearity, etc. Depending on the nature of the wireless device and the wireless signal that is being amplified, some of these criteria become more critical than others.
There are an increasing number of wireless communication standards that are employed by wireless communication devices. Current standards include Global System for Mobile communications (GSM), Enhanced Data rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), CDMA2000®, Long Term Evolution (LTE), and new standards are continually being developed. These standards provide for a variety of wireless signal specifications and formats, including different transmit level requirements, different modulation types, etc. For example, GSM employs Gaussian Minimum Shift Keying (GMSK), EDGE employs GMSK and 8-phase shift keying (8-PSK), UMTS employs Code Division Multiple Access (CDMA), etc. These different wireless signals in turn impose different requirements on whatever RF transmit amplifier is employed to transmit each of these wireless signals.
In particular, some of these wireless signals (8-PSK, CDMA) need to be amplified with a linear amplification characteristic, while for other signals (GMSK), the RF transmit amplifier can be operated in a saturated mode (i.e., in gain compression) without negatively affecting the transmit Bit Error Rate. In general, operating an RF amplifier in saturation can provide certain benefits over linear amplification. For example, operation in saturation is generally more efficient than linear operation, and therefore for a given RF output signal level, a saturated RF amplifier can reduce power consumption compared to a linear RF amplifier supplying the same output level. Also, in general an active device that provides a given output level in saturated mode can be made smaller than an active device that provides the same output level in linear operation. However, saturated amplification is inappropriate for certain signal formats because saturation can cause signal distortion that is undesirable for certain signals. This presents a set of design tradeoffs for a single RF amplifier that attempts to process some signals for linear amplification, and other signals for saturated amplification.
Furthermore, the various wireless communication standards operate in a variety of different frequency bands which can be different from country to country.
Also, some wireless communication devices exercise some form of control over the output power of the RF transmit amplifier. In particular, when the mobile communication device is operating somewhat far away from a base station, the device may operate in a “high power” mode wherein the RF output power level is set at or near its maximum value. On the other hand, when the mobile communication device is operating somewhat close to a base station, the device may operate in a “low power” mode wherein the RF output power level is set at a reduced level. Variations of this “dual-mode” operation are possible, for example, where the current battery voltage level and the remaining battery capacity are also taken into consideration when switching between operating modes. When the RF power amplifier is switched into the “low power” mode, it consumes less current from the battery than when it operates in the “high power” mode. Therefore, when the RF power amplifier is switched to the “low power” mode, the mobile communication device is able to conserve battery power and thereby extend the required time between charges.
So it is seen that there are a variety of different modes and different frequency bands in which RF transmit amplifiers operate in wireless communication devices.
Meanwhile, there is a desire for one wireless device to support several different wireless communication standards.
However, equipping a single wireless communication device to transmit a variety of different wireless signal formats in a variety of different frequency bands places a number of constraints on the RF transmit amplifier system. In particular, it is difficult to provide a single RF transmit amplifier that is optimized for transmitting all of the various wireless signals in all of the corresponding frequency bands.
One example of this increased integration, and the associated technical challenges imposed on RF transmit amplifiers, is the evolution of GSM wireless communication devices.
The first second generation (2G) devices in Europe were single-Band GSM wireless communication devices using the GMSK signal format operating in the 890-915 MHz GSM band (P-GSM-900). These devices typically used duplexers between the antenna, and the transmit and receive signals.
Subsequently, GSM was expanded to the 880-915 MHz Extended GSM band (E-GSM-900). The extended band was provided to improve capacity. However, the increase in bandwidth produced a more complicated RF transmit amplifier design. Also, the closer spacing between transmit and receive channels caused higher transmit noise in the receive band.
The development of dual-band devices added the 1710-1785 MHz DCS-1800 frequency band to the GSM-900 band. As a result, two separate RF transmit amplification chains were required, one for each band. The requisite doubling of components, especially filters, was undesirable given the expectations of simultaneous reductions in the size of wireless devices. Also, in these dual-band wireless devices duplexers were replaced with transmit/receive (T/R) switches.
Tri-Band GSM devices added the 1850-1910M Hz PCS band, wherein a GMSK signal is transmitted in any of the three frequency bands. Adoption of GSM in the United States PCS band expanded the role of the high-band RF transmit amplifier for dual-band applications. The bandwidth of the high band RF transmit amplifier increased from 4% to 11%, complicating the RF transmit amplifier design. Also, the difficulty of switching input transmit bandpass filters forced a more stringent specification on output noise power from the transmit amplifier.
Adoption of GSM in the United States expanded to the 824-849 MHz cellular band, leading to Quad-Band devices employing GMSK in four different frequency bands. As a result, the bandwidth requirements for the low-band RF transmit amplifier increased from 3% to 10%, complicating the RF transmit amplifier design. More stringent noise power specifications were also required.
Initial adoption of EDGE (which employs GMSK and 8-PSK) by Europe and eventually by the U.S. and other nations forced wireless communication device manufacturers to create the first multi-band multi-mode (MBMM) wireless communication devices. Several variants of RF transmit amplification solutions exist for these devices, including a 2-mode RF transmit amplifier, a single-mode polar RF transmit amplifier, and a single-mode linear RF transmit amplifier.
The 2-mode RF transmit amplifier operates in a saturated mode for GMSK, and in a linear mode for EDGE. In general, 2-mode RF transmit amplifiers are optimized for GMSK signals, and are sub-optimal for EDGE signals.
Single-mode polar RF transmit amplifiers offer software-programmable radio functionality and also address multi-burst problems that cause a failure of the GSM system power/time mask, and are seen on earlier devices that combine the GMSK and EDGE signals in a single RF transmit amplifier.
Single-mode linear RF transmit amplifiers are also software programmable and improve linearity for EDGE signals, at the expense of GMSK performance.
In parallel with the development of GSM/EDGE systems, competing CDMA-based systems (n-CDMA and W-CDMA) were developed. GSM and n-CDMA offer the best support for low-bandwidth voice applications, while W-CDMA best supports the burgeoning market for wireless data for mobile internet access, file sharing, productivity applications, and peer-to-peer data communications. EDGE can support both voice and data, but neither optimally as the current draw is too high for voice, while the bit rate too slow for data.
One system cannot adequately support all applications, forcing wireless communication device manufacturers to support several modes in a given device which can be selected for optimal performance based on the immediate application (e.g., GMSK or n-CDMA for voice; W-CDMA for data; EDGE for voice or data—based on capacity or network availability). Convolved with the need to cover virtually all available bands, what is developing are true MBMM wireless communication devices. Expanding upon the evolution of GSM, these MBMM wireless communication devices represent the next evolutionary phase, providing operation with GMSK/EDGE/W-CDMA signals, worldwide frequency band coverage, etc. In particular, what is generally desired is quad-band operation with GMSK/EDGE signals, together with operation with W-CDMA signals in from one to five (or more) bands, depending on the desired flexibility of the wireless device. Also, n-CDMA support in the W-CDMA paths is sometimes desired for further interface flexibility.
Several potential options exist to address the need for RF transmit amplification in these MBMM wireless devices. These options include: (1) a single RF transmit amplifier; (2) a single RF transmit amplifier with a DC-DC converter and a distribution switch; (3) dual RF transmit amplifiers with a distribution switch and a DC-DC Converter; (4) four RF transmit amplifiers; and (5) four RF transmit amplifiers with a distribution switch on the linear ports.
However, all of these solutions present their own problems or drawbacks.
A single RF transmit amplifier (Option 1) requires an amplifier operating over an extremely wide bandwidth and able to address the power level differential between low bands and high bands, and between W-CDMA and GMSK. These requirements make this potential solution untenable. This configuration also would require a tunable duplexer, which are not commercially available, for multi-band UMTS support.
With a single RF transmit amplifier with a DC-DC converter and a distribution switch (Option 2), the DC-DC converter allows adjustment of RF transmit amplifier output power according to the operating band and modulation. However, the difficulty of applying the DC-DC converter at high currents in dynamic conditions without spurious emissions or time mask problems complicates the DC-DC implementation, and the DC-DC converter adds cost and consumes circuit board area. The distribution switch solves the issue of tunable duplexer availability, but the switch is not desirable for GMSK or EDGE signals due to increased transmit chain loss.
With dual RF transmit amplifiers with a distribution switch and a DC-DC Converter (Option 3), the bandwidth requirement placed on each RF transmit amplifier is reduced to practical levels. However, this configuration has high GSM or EDGE transmit chain losses due to the distribution switch.
With four RF transmit amplifiers (Option 4), the GSM or EDGE vs. W-CDMA power level discrepancy problem can be eliminated by transmitting the different signals through separate, optimized RF transmit amplifier chains. This approach also eliminates the need for transmit switch for GSM or EDGE signals. However, there is a cost increase due to having four separate RF transmit amplifiers, and this configuration is limited to one band per UMTS RF transmit amplifier.
Having four RF transmit amplifiers with a distribution switch on the linear ports (Option 5) allows multi-band support for linear ports similar to option (2) for each RF transmit amplifier. However, again, there is a cost increase due to having four separate RF transmit amplifiers.
What is needed, therefore, is an RF signal amplification device that can be separately optimized for amplifying and transmitting wireless signals conforming to a variety of different standards in a variety of different frequency bands.